Mass Defect MCQs for NEET

The actual mass of the nucleus is lesser than the sum of the individual mass of the nucleons because energy is ejected when the nucleus is created. The mass that is missing from the final nucleus is called the mass defect. Once the value of mass defect is known, the nuclear binding energy can be calculated using the formula.


Here Δm is the mass defect.

Q1: Radioactive material A has a decay constant 8λ and material B has a decay constant λ. Initially, they have the same number of nuclei. After what time, the ratio of the number of nuclei of material B to that A will be 1/e?

  1. 1/λ
  2. 1/7λ
  3. 1/8λ
  4. 1/9λ

Answer: (b) 1/7λ

Q2: The half-life of a radioactive substance is 30 minutes. The time (in minutes) taken between 40% decay and 85% decay of the same radioactive substance is

  1. 15
  2. 30
  3. 45
  4. 60

Answer: (d) 60

Q3: If the radius of the 13Al27 nucleus is taken to be RAl then the radius of 53Te125 nucleus is nearly

  1. (53/13)1/3RAl
  2. 5/3 RAl
  3. ⅗ RAl
  4. (13/53)RAl

Answer: (b) 5/3 RAl

Q4: A nucleus of uranium decays at rest into nuclei of thorium and helium. Then,

  1. The helium nucleus has more kinetic energy than the thorium nucleus
  2. The helium nucleus has less momentum than the thorium nucleus
  3. The helium nucleus has more momentum than the thorium nucleus
  4. The helium nucleus has less kinetic energy than the thorium nucleus

Answer: (a) The helium nucleus has more kinetic energy than the thorium nucleus

Q5: The binding energy per nucleon of 3Li7 and 2He4 nuclei are 5.60 meV and 7.06meV respectively.

In the nuclear reaction 3Li7 + 1H1 ➡️ 2He4 +2He4+Q, the value of energy Q released is

  1. 19.6 MeV
  2. -2.4 MeV
  3. 8.4 MeV
  4. 17.3 MeV

Answer: (d) 17.3 MeV

Q6: Calculate the mass defect(amu/atom) for a 37Cl atom for which the actual mass is 36.966 amu.

  1. 0.341 amu
  2. 0.388 amu
  3. 0.623 amu
  4. 0.264 amu

Answer: (a) 0.341 amu

Q7: The mass defect for an isotope is 0.410 amu/atom. Calculate the value of binding energy.

  1. 1.23 x 1020 kJ/mol
  2. 2.23 x 103 kJ/mol
  3. 3.69 x 1010 kJ/mol
  4. 3.69 x 1013 kJ/mol

Answer: (c) 3.69 x 1010 kJ/mol

Q8: Which isotope below has the highest binding energy?

  1. 4He
  2. 16O
  3. 32S
  4. 55Mn

Answer: (d) 55Mn

Q9: Which of the following sentences is incorrect?

  1. Nuclear binding energy is the energy released in the formation of an atom from subatomic particles
  2. It the nucleus was formed from initially separated protons and electrons then the amount of matter converted into energy was called Mass defect.
  3. The total mass of protons and electrons in the atom.
  4. The most stable nuclei are molecules with the highest binding energy.

Answer: (c) The total mass of protons and electrons in the atom

Q10: A nucleus has a mass number A, atomic number Z and a binding energy B. The mass of neutron and proton are mn and mp respectively and cis the speed of light.

Which of the following expressions correctly represents the mass of the nucleus?

  1. (A – Z)mn+Zmp-B/c2
  2. (A – Z)mn+Zmp+ B/c2
  3. (A + Z)mn+Zmp– B/c2
  4. (A + Z)mn+Zmp+ B/c2

Answer: (a) (A – Z)mn+Zmp-B/c2

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